We are currently concentrating our efforts on cloning the gene for hypoxanthine phosphoribosyltransferase (HPRT--E.C.2.4.2.8). Our goal is to develop methods to examine the DNA of normal and mutant forms of the HPRT gene. Our specific aims are: A. to develop an assay for human HPRT messenger RNA; B. to partially purify the human HPRT mRNA approximately 100-fold; C. to prepare cDNA to the enriched HPRT mRNA; D. to prepare a "library" of these cDNA clones in PBR322; E. to identify the plasmid(s) containing cDNA specific for HPRT; and F. to use the HPRT cDNA plasmid(s) as a probe to identify the HPRT gene in a whole genomic library of human DNA.